Estimates the number of characters available for input in the associated character sequence. underflow() is guaranteed not to return Traits::eof() or throw an exception until at least that many characters are extracted.

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The number of characters that are certainly available in the associated character sequence, or -1 if showmanyc can determine, without blocking, that no characters are available. If showmanyc returns -1, underflow() and uflow() will definitely return Traits::eof or throw.

The base class version returns ​0​, which has the meaning of "unsure if there are characters available in the associated sequence".